There are several examples of miniature electromechanical devices equipped with movable microplatforms with lateral dimensions from a few micrometers to several thousands of micrometers. The pivoting along one or multiple axis, and vertical or horizontal displacement of these microplatforms is possible due to various types of hinge members including torsional and flexure hinge types. The movement of the microplatforms is induced typically via electrostatic, magnetic or thermal mechanisms.
The functions performed by these microdevices equipped with movable microplatforms include phase and amplitude modulation of visible, UV or IR radiation beams as well as mechanical protection with open and close functions of specialty microsystems. In particular, the microdevices used for radiation beam manipulation are often called micromirror devices and find multiple applications in telecom for laser beam attenuation and switching as well as in projection imagers as the spatial light modulators.
In optical applications in particular, it is extremely important to provide movable microplatforms that are free of mechanical deformations and defects, with a residual radius of curvature of many meters and surface roughness better than λ/10, where λ is the wavelength of radiation used.
There are typically two methods used for manufacturing individual and arrays of microdevices equipped with movable microplatforms. The first most commonly used method makes use of a thin film technology developed for manufacturing integrated circuits (ICs), while the second method typically involves micromachining silicon wafers and epitaxial silicon films by a deep etching technique.
Known in the art is U.S. Pat. No. 6,025,951 by N. R. Swart et al, which discloses a micromirror device equipped with a flexure hinge permitting to tilt as well as to displace vertically a microplatform via the electrostatic interaction. Methods for forming individual or multiple microdevices are also described. These methods make use of the thin film technology developed for manufacturing ICs.
The biggest drawback of the microplatforms produced by the thin film technology is a residual stress in the deposited thin films as well as a mismatch between stresses in films made of different materials. This produces deviations from the platform flatness required and thus adversely affects the microdevice performance. Typically, the residual stress in thin films scales inversely with the film thickness. However, the thin film technology yields a film with a thickness not exceeding a few micrometers and thus the stress reduction is not sufficient. Thin films also have rough surfaces, which contributes to unwanted scattering of radiation illuminating the microplatform.
Also known in the art is U.S. Pat. No. 4,317,611 by K. E. Petersen, which discloses a torsional-type optical ray deflection apparatus produced out of a pair of etched plates, one of which is single crystal semiconductor material such as silicon, and the other is a suitable insulating material such as glass. A pivoting microplatform equipped with torsional hinges is made by anisotropic etching through a silicon wafer. It is then bonded to a glass plate equipped with suitable metal electrodes for electrostatic activation of the microplatform.
Also known in the art is U.S. Pat. No. 6,044,705 by A. P. Neukermans et al, which discloses a different approach to manufacturing of pivoting microplatform devices making use of a stress-free semiconductor layer of silicon. The epitaxial silicon layer with a thickness ranging from less than one to tens of micrometers is grown on the etch stop layer deposited on the Si wafer.
Also known in the art, there are U.S. Patent Application 2001/0044165 A1 by S. B. Lee et al. and U.S. Pat. No. 6,353,492 B2 by R. W. McClelland, which disclose pivoting microplatform devices made entirely from monocrystalline silicon wafers by the deep etching technique.
Application of silicon in the form of polished wafers or epitaxial layers deposited on silicon wafers allows to alleviate some limitations associated with the thin film technology. Silicon in this form is light, strong and stiff, yielding rigid microplatforms with low moment of inertia. It also yields a wide range of microplatform thicknesses from tens to hundreds of micrometers. Silicon wafers can be polished to provide excellent surface quality (i.e. very low roughness) and flatness. They will reflect radiation effectively when covered with appropriate coatings. On the other hand, patterning of silicon wafers may be a limiting factor in terms of minimum feature size or geometrical form especially if the anisotropic wet etching technique is applied. Moreover, the microdevices equipped with movable monocrystalline silicon microplatforms are typically of hybrid construction. They consist of at least two different parts, one comprising the microplatform and the other one performing a function of the supporting base equipped with electrodes and contact pads for actuation of the microplatform. Fully monolithic devices are difficult to implement and are mostly formed by the competing thin film technology.
The following United States Patents disclose other devices suffering from the same drawbacks as described above: U.S. Pat. No. 5,083,857 (L. J. Hornbeck); U.S. Pat. No. 5,212,582 (W. E. Nelson); U.S. Pat. No. 5,233,456 (W. E. Nelson); U.S. Pat. No. 5,312,513 (J. M. Florence et al.); and U.S. Pat. No. 5,789,264 (J. H. Chung).